Design and characterization of metamaterial antenna for mobile handset applications

Abstract

A metamaterial antenna with slots is investigated in this doctoral work for a new type of antenna for mobile application, which provides wide bandwidth, miniature size and high gain. This antenna exhibits handedness, left-handedness and right-handedness in the resonant frequency with good radiation characteristics with Composite Right/Left Handed Transmission line (CRLH TL) Split Ring Resonator (SRR) and PATCH as its central elements. The conventional microstrip antennas have a conducting patch printed on a grounded dielectric substrate and operate as resonant cavity elements. This operation, however, leads inherently to narrow impedance bandwidth, which is a barrier for microstrip antennas to be used for wireless communications. Hence, they are generally used for multi-standard wireless communication systems as they do not offer multi-band performance. Antennas for next generation wireless communication systems are required to exhibit resonance in multiple bands that are not harmonically related in order to accommodate multiple wireless standards within the same device. Additionally, they are required to have a compact form-factor and relatively high radiation efficiency. Literature related to the compact antennas intended for use in handsets and other small wireless devices were studied in detail in the chapter -2. The nutshells proposed within the antenna, CRLH TL, SRR and Patch metamaterial concept were also surveyed in some of the relevant recent studies. Advantages of presence vi of slot and feeding techniques employed for the metamaterial antenna for mobile applications are also investigated. On the basis of literature survey, an novel prototype antenna, which provides wider bandwidth, miniaturized size and high gain along with the above cited concepts is proposed in this work as the final design. The research described in this thesis is concerned with the analysis and design of mobile handset antenna types, based on the left-handed transmission line, S-SRR and patch with slot approach. The left handed transmission line concept can be extended to construct reduced-size planar antenna in the commercial frequency band which is thought to be a feature unique to these antennas. Efficiency is a key parameter of left handed antennas when split ring resonators are considered. A study of the efficiency of miniaturized mobile antennas loaded with a left-handed transmission line is described specially, and is compared with those of the conventional PCB antennas. Available literature shows the slot in the antenna helps in bandwidth enhancement. The bandwidth enhancement of the proposed antenna is achieved by unequal sizes of slots created on the radiating patch of various sizes. The major aim of the third chapter of this thesis is to demonstrate the scheme of design development of a wideband handset antenna, for mobile range covering GSM, PCS, DCS and WIBRO bands. The design development is presented in terms of a flow chart. The prototypes have PCB type structures printed on both sides, covering frequency of mobile applications with unsymmetrical cells presented. The first part of this chapter deals with the dimensions of three antenna structures, and their simulation to obtain best vii optimized results. Of the three antennas, while the first antenna has substrate of FR4, the second and the third have substrate of RT Duroid. The three antennas differ in structure and dimensions. The first of the prototypes has probe feed printed on both sides. The geometry of this two sided printed circuit CRLH -TL metamaterial antenna is discussed in some detail, mentioning the dimensions and copper conducting plane and substrate. The first design sample with FR4 substrate and coaxial feed is presented with FR4 (εr=4.4), with dimensions of 40×80×1 mm3. Antenna of miniaturized size, with dimensions of 40×60×0.762 mm3, with a printed type on a substrate of RT Duroid 5880LZ (εr=2.2) and edge feeding is subsequently proposed. The subsequent prototype designed has the same dimensions, and differs by the presence of slots of different sizes in the unsymmetrical patches of front side. The third chapter later discusses the fabrication of these three antennas; 1st through chemical etching of the antenna which has FR4 substrate, 2nd through chemical etching of the antenna which has RT Duroid substrate and without slot, and the 3rd through mechanical etching, which has RT Duroid and slot in the patches. Of the two etching methods used, the mechanical one is found to be better and adapted, with reasons discussed. Simulations for the mentioned three prototypes are studied and compared by the output parameters, listed by, Return loss, Radiation Pattern, Gain, VSWR and axial ratio. Optimized simulation results are determined followed by characterization, for measurement of return loss through vector network analyzer, radiation pattern through anechoic chamber to measure HPBW hence gain and directivity. Design consists of unsymmetrical viii cells with combination of different sized patches to resonate in the frequency range mobile applications. This dimensional variation in turn makes antennas to resonate with different central frequencies. The improvement in characteristics is observed right from the first prototype, progressing to the third antenna, along with increased number of center frequencies. Analytically ‘equivalent’ antenna is determined for all the prototypes. Considering with the ‘equivalence’ of individual elements, for all the main elements including CRLH, S-SRR, and PATCH with slot of the prototype having slots on one side, the equivalent circuit of an antenna is drawn. The input impedance for analysis is derived from the overall circuit according to current from feed point to ground. The reactance component of input impedance is then plotted to obtain the resonating frequency. This parameter is discussed in section 4.4. An extensive list of output parameters is discussed in chapter 4 for the antennas, which were fabricated according to the relevant existing literature. Of them, parameters like Return loss, Radiation pattern, Gain measurement and Input impedance plot are discussed in detail in chapter 4. The resonant frequency of the antenna is inferred from the plot of return loss. The simulation results show that the antenna resonates from 1.58 GHz to 1.95 GHz. The measurement results of prototype were shifted towards right, with -10dB frequency values 1.65 GHz to 1.99 GHz. The shift of the two values is attributed to the fabricational inconsistencies, like discontinuities at the outer contours of the conducting patch lines on the substrate. This antenna covers PCS and DCS bands. ix The resonant frequency of the 2nd antenna prototype antenna is obtained from return loss plot. The simulation results show that the antenna resonates from 1.95 GHz to 2.45 GHz. The tested result of prototype was shifted towards left with 1.9 GHz to 2.3 GHz. The shift of the two results can be observed by fabricational inconvenience faced in the discontinuities at the outer contours of the conducting patch on the substrate. The low-frequency resonance at 1.9 GHz, the current distribution is good enough to get wider bandwidth. This antenna is covering PCS and DCS and BLUETOOTH. Limitation of this antenna is its feeding increasing thickness of mobile or wireless sets. The 3rd antenna prototype which is a slot loaded metamaterial antenna exhibits distinct resonances at 1.91 GHz, 2.4GHz 3.35GHz and 4.62 GHz respectively. Return loss responses explains the four resonances that lie within the S11 with value equal to −10 dB line can be clearly seen. It can be observed that in all three cases the antenna has a dipolar pattern, exhibiting a linear electric field polarization in the y-direction. The 10% shift towards right is observed due to the losses of connector and fabricational anomalies. The sizes and placement of the slots etched on patches are defined, such that a multi-band version of the antenna can be achieved with a large lower frequency bandwidth. The prototype with slots on the radiating patches is proved to be resonating more number of bands compared to the prototype which doesn’t have slots on the front side. The mechanical etching method employed for final antenna fabrication also proved to be better in terms of accuracy, with respect to chemical etching used for antenna without slots. The resonating bands of antenna are discussed considering x the -10dB power of Return loss response of individual antennas. Radiation patterns, Gain, Voltage Standing Wave Ratio(VSWR) and Axial ratio are also plotted for the all three designs. Thus, a very good agreement between the design (simulation) and experimental results was obtained. Moreover, a comparison of the measured and predicted antenna output efficiencies, as functions of frequency at 0 dBm input power done for both the simulated and measured results. Some differences in the results for the simulation and measurements are observed. Next section in chapter 4, deals with the rectangular radiation pattern which is measured through anechoic chamber, by obtaining half power beam widths (HPBW) for Electric and Magnetic fields. The gain and directivity are further measured from this. It is clear that amongst the gain obtained for the three antennas, prototype based on RT Duroid and with slot, has the highest gain at resonant frequency. The last section chapter 4 deals with the response of reactance of input impedance. It also confirms the values of resonating frequencies as same as those obtained from the return loss response at 1.9GHz of the antenna with slots. The complexity in the designing of transceivers, which are vital for communications, are drawn out clearly from the present work. With increased sophistication in personal locator boards, there is a growing need for flexible high-speed communication with the ‘outside’ devices. Today communication is done by wireless sets which are embedded into MICS, and are used in numerous applications. Also, the present work throws up the possibility of using the 2.45 xi GHz band for mobile communications. Although, this band has the drawback of being heavily used by other applications, such as wireless computer networks and in Bluetooth applications. Thus, both theoretical limits and practical designs of the relevant antennas are described and discussed in detail in this work.